This invention relates to a method for welding stainless steel, and more particularly it is concerned with a welding method, suitable for use in places where no recourse can be had to solution heat treatment for preventing stress corrosion cracking, for joining tubular members of low carbon stainless steel of high corrosion resistance adapted to handle a fluid of high temperature and high pressure, without causing stress corrosion cracking in the welding heat affected zone of the piping produced.
There have in recent years been occurrences of stress corrosion cracking in the grain boundary of the welding heat affected zone of piping of SUS304 for the primary system of nuclear power plants of the boiling-water reactor type. Such stress corrosion cracking is produced, as shown in FIG. 1, in a portion 4 of material in which a high tensile stress 1 exceeding 0.2% proof stress, a presence of chromium-depleted-zone 2 (sensitization of the material) produced along the grain boundary in the welding heat affected zone and a corrosion environment 3 including the presence of dissolved oxygen act simultaneously. In a welding process heretofore used for joining tubular members of SUS304 wherein natural cooling is effected without performing forced cooling as by passing cooling water through the inside of the tubular members, shrinkage of the material occurs as a result of a butt weld 6 as shown in FIG. 2(a). This induces high tensile stress [represented by a line A in FIG. 2(b)] as high as several tens of kg/cm.sup.2 on the inner surfaces of the tubular members 5A and 5B of SUS304. In FIG. 2(b), the TS side is a tensile stress zone, and the CS side is a compressive stress zone, and a line B indicates a 0.2% proof stress level. The chromium-depleted-zone 2 is produced in a welding heat affected zone 7 of the tubular members 5A and 5B. When a corrosive fluid flows through the inside of the tubular members 5A and 5B in which the high tensile stress 1 and chromium-depleted-zone 2 have been produced, there is a high risk of occurrence of stress corrosion cracking in the welding heat affected zone 7 of the tubular members 5A and 5B.